An HVDC installation usually has two converter stations. The converters in one station normally operate as rectifiers and the converters in the other station as inverters. If a ground fault occurs on the dc line in such an installation, this is detected by a line protection device. Such protection devices are well known. A typical line protection device makes use of two criteria for detection of a ground fault. The derivative part of the line protection device indicates a ground fault if the direct voltage decreases to a low level but at a certain speed (possibly it is also required that the direct current increases). The level part of the protection device indicates a ground fault if the direct voltage falls below a low limit value for a certain minimum period of time. This time has to be chosen so low, for example 150-300 ms, that unjustified ground-fault indications are not obtained during normal switching processes or during such other disturbances, for example loss of the alternating voltage of one of the stations, which also give rise to a low direct voltage.
When the line protection device indicates a ground fault on the dc line, the rectifier is controlled in a known manner to inverter operation, and the current and voltage of the line are reduced to zero. After a certain time, a restart attempt is then made. This time is so chosen (e.g. 100-200 ms) that arcs which occur during temporary ground faults on an overhead line should have had time to become extinguished and deionized. During a restart attempt, the voltage and current of the line are increased. If the fault no longer remains, normal operation is resumed. If the fault remains, the protection device reacts again. Usually, a certain number of restart attempts are made, whereupon, in case of a remaining fault, the operation of the installation (or the pole concerned) is interrupted.
Installations of the above kind are previously known from, for example,
A dc line in an HVDC installation may comprise an overhead line or a cable or, perhaps more often, partially an overhead line and partially a cable. Ground faults on an overhead line are often temporary. In the event of such faults, it is desirable to resume normal operation as quickly as possible, and this can be done with minimum disturbance of the operation in the manner described above. Ground faults on a cable, on the other hand, are normally permanent. Making one or more restart attempts during such a ground fault is ineffective. The restart attempt or attempts only delay(s) the definite interruption of the operation. Further, during the restart attempts repeated high currents are fed through the location of the fault, each time for a relatively long time (such a long time that the line protection device has time to trip). This entails, on the one hand, a risk of additional damage of the cable, and on the other hand, heavy disturbances in alternating-voltage networks connected to the installation, and especially in weak networks.